Heretofore, a high zirconia fused cast refractory comprising at least 80 mass % of ZrO2 as a chemical component has been used as a refractory for a glass melting furnace. A high zirconia fused cast refractory has high corrosion resistance against molten glass and low contamination properties, and has been frequently used for a portion in contact with molten glass in a glass melting furnace. Such a high zirconia fused cast refractory is constituted by a large amount of zirconia crystal grains and a small amount of matrix glass filling spaces among such crystal grains.
In recent years, a demand for further raising the temperature for melting glass has been increasing, and even a high zirconia fused cast refractory may not sometimes adequately satisfy the corrosion resistance. Therefore, a refractory having a further higher corrosion resistance is desired for the glass melting furnace.
In a refractory utilizing zirconia crystals, in order to increase the corrosion resistance to high temperature molten glass, the content of ZrO2 in the refractory may usually be increased, and various studies have been made for high zirconia fused cast refractories. As such high zirconia fused cast refractories, specifically high zirconia fused cast refractories having the content of ZrO2 increased to a level of at least 90 mass %, further to a level of at least 95 mass % are known (e.g. Patent Documents 1 to 4).
In a high zirconia fused cast refractory having the content of ZrO2 increased to a level of at least 95 mass %, the matrix glass is 5 mass % at the maximum i.e. its proportion to the entire refractory is small. However, the physical properties of the matrix glass contribute substantially to the physical properties of the refractory, e.g. reduction of residual volume expansion (hereinafter referred to simply as residual expansion), suppression of leakage of matrix glass, or prevention of cracking during the production. Therefore, in a high zirconia fused cast refractory, optimal adjustment of the glass composition of the matrix glass, particularly adjustment of the contents of very small amount components, is important.
Further, with a high zirconia fused cast refractory having the content of ZrO2 increased to a level of at least 95%, the refractory was susceptible to cracking, and it was difficult to produce it in a size useful as a furnace material for a glass kiln. Usually, a high zirconia fused cast refractory is produced by melting a raw material for the refractory at a high temperature of at least 2,500° C., followed by cooling in a casting mold. As the zirconia content in the refractory becomes high, the melting temperature becomes higher, and cracking is likely to result when a large size refractory is to be produced.